Detailed analysis of N-channel depletion MOS transistor as active resistance

N-channel depletion type as active resistor

When N-channel depletion type is used as an active resistor, its connection mode (ie, gate-source short-circuit) and V-I characteristics are shown in Figure 2.1-4 and b respectively.

It can be seen from the figure that when the tube enters the saturation zone, the impedance at both ends of the drain and source is high impedance.

With Figure 2.1-5 AC small signal equivalent circuit, the equivalent impedance ro of the drain-source terminal can be obtained.

When the voltage between the substrate and the source VDS=0, the figureall equal to zero（）, There are

When VDS≠0, then

It can be seen from equations (2.1-6) and (2.1-7) that the active resistance of the N-channel depletion MOS transistor is larger than the active resistance of the above-mentioned enhanced MOS transistor. Therefore, when the N-channel depletion MOS tube is used as the active load of the amplifier, it has a higher voltage gain.

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